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Examination of piston accuracy (6.5 Grendel)

I posted this quick review over at M4carbine but now the Grendel site is here this is more appropriate

During the past few months I have been slowly examining the effects of using a piston type operating mechanism on the accuracy of 20" DMR type build. The baseline rifle for the study is our existing GDMR which houses a 20" Satern barrel with rifle length operating system in a monolythic upper, in which the railed handguard forms an integral part of the whole reciever. The gas system uses a conventional gas tube with a non adjustable gas block and a port size of 0.093" which is nicely balanced for most loads. The barrel profile is medium heavy at 0.980" dia under the handguards with 8 off 3/16" flutes. Typical fleet data from building and testing these rifles during production shows the accuracy to be around 0.5 inches at 100 yards for a 10 round group, testiment to the Satern barrel.

The corresponding piston operated rifle came into being due to the duty cycle of one of the customers. Extensive use of suppression accelerates fouling and heat transfer so a piston design was requested. The rifle shares a common chassis and gas system length, although the barrel fluting is heavier to help with weight. Gas port diameter increases to 0.106" to balance the Adams Arms gas piston used in this build with most ammunition.

The testing was started with the preconception that the gas piston would open the group sizes. It was therefore surprising to see that accuracy was identical to that seen in the direct impingment units. Recoil although subjective felt somewhat smoother and more straight line. Responding to the possibility that the shift in accuracy was being masked by the barrel quality, an identical unit was made using a Begara button rifled blank. Again the rifle shot exceptionally well and it was clear that the piston mechanism, although it imposes off axis loadings has little effect on the accuracy of this rifle layout. To verify accuracy was examined out to 600 yards.

It is hypothesised that for the 6.5 grendel, the piston type design, when restrained within a suitably ridgid reciever has no negative effects on the accuracy of the host weapon. Timing is still set by the port position and unlock timing is adjusted by the same.

This is a very rough summary of work to date and more comprehensive testing is required. Additionally it is proposed to build a series of shorter barrel weapons to se how these respond.

I vaguely remember reading a report (don’t remember where) a good while back by someone reporting accuracy degradation from a piston system. However, it was not a lot and it was a standard A-4 pattern rifle. Other than that one, I have never seen anything else from someone who actually tried it.

I wonder what would happen if you went the other way and didn't drill a hole at all. What effect would making it a bolt action have on accuracy? I've always wondered how much accuracy if any is lost using semi-automatic actions.

There was a lengthy post about the subject by another member (I forget who?) on the old forum. The underlying theory (as best I recall) had to do with reciprocating mass. A DI gun has all of the reciprocating mass (bolt, bolt carrier, buffer, action spring, etc) traveling in direct line with the bore axis, whereas piston guns have at least a significant portion of their reciprocating mass above the bore axis. This will cause additional off axis "upset" during the time the weapon is cycling. If you watch slow motion videos of some weapon systems you can actually see the torque and vibration that this causes. A slow motion of the AK looks like a train wreck on some of them. You can actually see lots of receiver flex and other stuff going on. How much this actually applies to the AR platform I have no idea, but I haven't seen any match guys running to the piston system for their match guns. It could be that the whole issue is overblown, but maybe not also. If Bill A. (or others) can turn out accurate piston guns, then people will probably ease up on this prejudice.

How much this actually applies to the AR platform I have no idea, but I haven't seen any match guys running to the piston system for their match guns.

The 3gunners definitely prefer less reciprocating motion. Lightened bolt carriers with adjustable gas systems so there’s just enough energy to cycle the system and nothing more. The only 3gunners using piston systems that I can think of is the FNH team who use SCARs.

I'm pleased to see actual data about accuracy on piston vs direct impingement systems. I would also like to see data about the recoil impulse of similarly built rifles where the only difference is piston vs direct impingement.

I would be interested in both the amount, and duration of the recoil impulse, and the perceived feeling of the recoil impulse.
Realizing that various muzzle brakes affect the recoil impulse, I would also be interested to find out about how the three or four top brakes affect recoil impulse; in other words, how effective they truly are.

and is a comparison of the two methods keeping a lot equal. A dominate concept presented is impact of the piston on the bolt. However, I do not see much impact as the bolt and spring loaded piston appear to be in full contact before the trigger is pulled. The piston seems to push the bolt rearward when the gas gets behind it.

I enjoy this cleaner, cooler action in my LWRC rifles (4) which are accurate for me (especially the .308 Repr), but I am am having a bit of issue with my #2 6.5 Saber equipped by factory with Adams piston. (10% stove piping discussed in Troubleshooting). The piston designs are different, and the Adams bolt spring is still a mystery to me. But when it shoots it is a tack driver at 200 yards! But so does my direct impinge #1, dirt and all.

And my other piston rifles Scar 17 and FNAR have not missed a lick yet, nor have my Galil or ak-47. Now I have had a few issues with a pair of old M1 Garrands but cleaning the piston tube corrosion solved that.

First of all, hello, new to forum and posting in general. With that said if I offend or step on toes...it's due to ignorance. A quick background....served in the Army for the last ten years, recently separated, purchased a GPU from a custom builder. I have been shooting this rifle frequently, more than i probabaly should, but now that Hornady is making rounds, things are a lot more affordable for me. Without the clever use of high tech machinery, I am assuming that the barrel is now tuned. I have not noticed any shift in in POA/POI. I do not bench rest, I shoot iron sights from 25-500m. These ranges are familiar and comfortable to me, but outside of punching paper on the range, this weapon is formidable on the local white tail population, and bags elk no problem. After years of 5.56 and 7.62 it is a breath of fresh air to fire a high quality round for a change. We can quote numbers from magazines and manufacturers all day and it would be the blind leading the blind. Just about every post I have read in this forum has been people trying to convince other people who is more intelligent by quoting the best rendition of BC etc. I submit ( esp to new shooters and people new to the AR platform) that the majority of advice found on here is subjective. Use common sense, acquire the same sight picture, and as Bill A said they fly down range on their own. For all you zombie fighters and SHTF people might I suggest that you find a weapon system that you are comfortable with. Everything else is just talking. Someone can tell you how awesome the rig is and not be able to shoot well at all. At close range you can kill medium game with a .22, shot placement is your friend esp when fighting hordes of zombie deer. But enough fun, I chose the piston system because of the extreme reliability over DI rifles. I enjoy the fact that the bolt and carrier stay cool to the touch after many rounds have passed through. Clean up is easy...just a couple passes with a bore snake and everything is tits. I have plenty of real world experience in tactical shooting and for my money I rely on this platform and cartridge without hesitation. By the way for those interested, I'm not very good at Grendel trivia, so pls dont try, I can however hit what I aim at, and will go on record as saying that is a good thing.

I have a 556 with a LW barrel that shoots under 1 moa. I put a Osprey Defense piston on it. I thought I would loose accuracy but it was not the case. I did not even have to work up a new load for it. It is a different piston system then the adams.

Maybe I just got lucky. I have not been willing to put a piston on my grendel although I am still thinking about it.

I often wonder, though, the quality of builds and maintenance in failure rates.

I had to shoot something like 700 rounds over many months, without cleaning, to get my Grendel to have a misfeed.

Like I said in my first post on this thread, i chose a piston for my rifle because I truly believe in the operational quality of the system vs DI. The majority of the weapons systems I fired over the years have been military issued, some fairly old and not tended to properly. I have also owned several DI chambered for 5.56, and have always had problems with stove piping double feeding etc. I've tried dry lube, Rem oil you name it all to no avail. This is the first rifle I had built to my specifications and for a different caliber, with the micro slick and the piston system including the PWS compensator the felt recoil is approx the same to the 5.56 and the muzzle rise is all but unnoticeable. After years of 2nd, 3rd and lowest cost firearms use, I spent the money for this superior round, and have my first 'high class' AR. at a cost of 1500 for the upper alone it should be hahaha. I have put around 600-700rnds through it and only cleaned the piston tube twice, no lube. outside of brass fouling on the extractor, no other cleaning was needed. I have not had a single misfire/malfunction to mention. So in short I do agree with your assessment bwaites, but i also cant shake my opinion of DI systems. I am also currently considering a hydraulic buffer, my thought being that even though it is a semi auto rifle, it will all but eliminate recoil. But I am hesitant to do so because I am not confident that a factory load will expell enough gas to slide the bolt to the rear properly if the hydraulic buffer is installed. I am really curious to know what the you salty dogs think on this.

After years of 2nd, 3rd and lowest cost firearms use, I spent the money for this superior round, and have my first 'high class' AR. at a cost of 1500 for the upper alone it should be hahaha.

So If I understand correctly: you ran very cheap or poorly/not maintained DI uppers and they weren't reliable. Then when you spent $1500 on a piston upper it ran well. Ever consider that a quality DI upper would have also? The same goes for ammo if that wasn't addressed.

FWIW: Without even counting my own experiences-- I witness piles of quality DI guns run very well every time I go to qualify at work. Perhaps they were different from what you were using.

Am I understanding that there are issues with the Adams piston system specifically? I've had one sitting in the safe for over a year waiting for this build I'm starting, but I don't want to (figuratively) shoot myself in the foot by installing an inferior system, regardless of how much I paid for it.